1. Field of the Invention
The present invention relates to a permanent magnet having improved field quality and apparatus employing the same and more specifically relates to generally cylindrical permanent magnets cooperating with face plates to define a hollow enclosure within which cooperating apparatus components may be placed.
2. Description of the Prior Art
In magnetic instrumentation like mass spectrometers the magnet's size, weight, and precision are generally the parameters which determine mostly cost and performance of the instrument. Though new magnetic materials offered many opportunities to reduce size and weight, the increased requirements in precision on the other hand practically outweigh these benefits. Today's magnets often are still big in size and weight, and precision (mostly uniformity) is always an issue.
A dipole magnet is characterized by two magnetic poles, called north and south pole, between which a magnetic field is established. The simplest form is a bar magnet shown in FIG. 1.
Scientific and technical applications often require a uniform magnetic field within a certain volume that can be represented by parallel magnetic field lines. To approximate this field, different magnetic shapes are known which commonly bring the magnetic poles into opposite positions to form a gap in which inner area the field lines are more or less parallel. Simple forms are the horseshoe (FIG. 2) and the U-shaped magnet, and widely used is the H-shaped magnet shown in FIG. 3. An H-shaped magnet requires two flat magnets of cylindrical or rectangular shape, which are magnetized along the short axis. For an efficient backflow of the magnetic flux, a yoke made of soft steel connects the backsides of each magnet; thus the magnetic flux through the cross-section of the structure resembles the capital letter H which gave the magnet the name.
Though the H-shaped magnet represents one of the most efficient concepts the field in the gap shows imperfections in areas away from the center. Carefully shaped pole pieces can reduce the effect of fringing fields and extend the area of useful uniformity, but they cannot eliminate the fringing fields in principle. This is common for all magnets where the edges of the poles are free in air.
Ring magnets are well known in many applications—obviously new is the consideration of the special boundary conditions for the inside magnetic field. The ring magnet itself generates a magnetic field like a bar magnet, see FIG. 4. The smaller the inner diameter compared to its height, the more it resembles the bar magnet. A ring magnet closed with pole plates reveals an entirely different perspective for the same objective.
In spite of the foregoing known types of permanent magnets, there remains a real and substantial need for improved permanent magnets which can provide for enhanced uniformity of magnetic field, strength of magnetic field and reduced weight and cost of manufacture.